Direct part marking (DPM) has allowed workpieces to be directly marked, identified and traced to their origin, and its use has been growing in the automotive, aerospace, electronics, medical equipment, tooling, and metalworking industries, among many others. A typical DPM is a machine-readable, high-density, two-dimensional, matrix-type, optical code, e.g., the known DataMatrix or QR codes, although one-dimensional optical codes are also known. Each DPM code is comprised of multiple elements that are directly marked (imprinted, etched, molded, or dot-peened) on a workpiece. For example, an outer surface of a metal workpiece may advantageously be dot-peened with such sunken elements as hemispherical depressions; an outer surface of a plastic workpiece may advantageously be molded with such raised elements as hemispherical bumps; and a laser may be used to etch elements of different light reflectivity closely adjacent an outer surface of a workpiece. Other shapes for the elements, and other marking techniques, may also be used.
Despite the ability to control very tight specifications on element size, width, spacing and so on, the lack of sharp contrast of machine-readable optical DPM codes directly marked on metal, plastic, leather, glass, etc., workpieces has prevented traditional moving laser beam readers from electro-optically reading the DPM codes reliably. These moving beam readers emitted a laser beam, which reflected off the highly reflective, typically non-planar, metal or glass, workpieces as bright return light that saturated on-board photodetectors.
Instead of moving laser beam readers, solid-state imaging readers have been used to electro-optically read by image capture diverse indicia, including DPM codes. The imaging reader included a solid-state imager (also known as an imaging sensor) with a sensor array of photocells (also known as pixels), which corresponded to image elements or pixels over a field of view of the imaging sensor, an illumination assembly for illuminating the field of view with illumination light, an imaging lens assembly for capturing return ambient and/or illumination light scattered and/or reflected from a DPM workpiece in the field of view, and for projecting the return light onto the imaging sensor to initiate capture of an image of the DPM workpiece in the field of view, and a microprocessor for analyzing, processing and decoding the DPM code from the captured image.
Yet, the use of imaging readers, especially handheld readers, for reading DPM codes on workpieces has proven to be challenging. Each DPM code is relatively small, e.g., less than 2 mm×2 mm. The workpieces themselves often have complicated, i.e., non-planar, curved, reflective surfaces. Contrast between the DPM codes and their workpiece backgrounds, especially from their outer, reflective background surfaces, is still often less than desirable. Unlike machine-readable codes printed in one color (for example, black) on paper of another color (for example, white), DPM codes are typically read not by a difference in intensity of the return light between regions of different color, but by shadow patterns that are cast by the raised or sunken or etched elements. Ambient lighting conditions, however, are variable. Illumination from the on-board illumination assembly is directed at variable angles. Reflections from ambient light sources and the illumination assembly often appear in the field of view of the imaging reader as hot spots, glare, or specular reflections of intense, bright light that saturate the imaging sensor, thereby degrading reading performance.
The art has attempted to enable imaging readers to more readily read DPM codes. For example, U.S. Pat. No. 8,028,913 described the use of a diffuser to diffuse the illumination light directed to the DPM code, U.S. Pat. No. 7,201,321 described a technique to enhance the contrast of the DPM code, U.S. Pat. No. 7,726,572 described a digitization technique for digitizing the DPM code, U.S. Pat. No. 7,350,711 described the use of ambient light shields and filters to read the DPM code, U.S. Pat. No. 7,163,149 described the use of more than one illuminating light assembly to illuminate the DPM code, U.S. Patent Publication No. 2009/0218403 described an enhanced aiming technique to locate the DPM code, and U.S. Patent Publication No. 2006/0138234 described an extended dynamic range in which the DPM code could be read.
Yet, as advantageous as such attempts have been, there is still a need to enhance the readability of DPM codes by image capture in a more cost-efficient and rapid manner without resorting to extra hardware that increases cost and weight, and without resorting to extra software that slows reading performance.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.